講師:Prof. Pengcheng Dai  (Rice University)
日時:2023年7月20日(木) 14:00~
場所:東京大学 柏キャンパス基盤棟 物質系講義室(2B6)

タイトル:Competing itinerant and local spin interactions in kagome metal FeGe

Two-dimensional kagome metals consisting of corner-sharing triangles offer a unique platform for studying strong electron correlations and band topology due to its geometrically frustrated lattice structure. The similar energy scales between spin, lattice, and electronic degrees of freedom in these systems give rise to competing quantum phases such as charge density wave (CDW), magnetic order, and superconductivity. For example, kagome metal FeGe first exhibits A-type collinear antiferromagnetic (AFM) order at $T_{\rm N}\approx 400$ K, then establishes a CDW phase coupled with AFM ordered moment below $T_{\rm CDW}\approx 100$ K, and finally forms a $c$-axis double cone AFM structure around $T_{\rm Canting}\approx 60$ K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure at temperatures well above $T_{\rm Canting}$ and $T_{\rm CDW}$ that merge into gapped commensurate spin waves from the A-type AFM order. While commensurate spin waves follow the Bose population factor and can be well described by a local moment Heisenberg Hamiltonian, the incommensurate spin excitations first appear below $T_{\rm N}$ where AFM order is commensurate, start to deviate from the Bose population factor around $T_{\rm CDW}$, and peaks at $T_{\rm Canting}$, consistent with a critical scattering of a second order magnetic phase transition, as a function of decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order. The temperature dependence of the incommensurate spin excitations suggest a coupling between spin density wave and CDW order, likely due to flat electronic bands near the Fermi level around $T_{\rm N}$ and associated electron correlation effects.